Abstract

N-Sulfinylaniline (C₆H₅NSO), systematically named (phenylimino)-λ⁴-sulfanone, represents a significant organosulfur compound within the sulfinylamine class. This straw-colored liquid exhibits a density of 1.236 g/mL and boils between 88–95 °C at reduced pressure (17–20 mmHg). The compound demonstrates notable dienophilic character and serves as a versatile ligand in coordination chemistry. Structural analysis reveals a nearly planar configuration with a C–S=N=O dihedral angle of –1.60°, closely resembling both sulfur dioxide and sulfur diimide in electronic structure. N-Sulfinylaniline finds applications in organic synthesis, particularly in Diels-Alder reactions, and functions as a building block for more complex sulfur-nitrogen compounds.

Introduction

N-Sulfinylaniline occupies an important position in organosulfur chemistry as a representative of sulfinylamines, compounds characterized by the R-N=S=O functional group. This class of compounds bridges the chemistry of sulfoxides and imines, exhibiting unique reactivity patterns derived from the polarized S=N bond. The compound was first characterized in the mid-20th century following developments in sulfur-nitrogen chemistry. Its synthesis from aniline and thionyl chloride represents a straightforward route to sulfinylamines, though careful control of reaction conditions is required to prevent over-reaction to sulfonyl derivatives. The compound's significance extends to both fundamental studies of bonding in S-N systems and practical applications in synthetic organic chemistry.

Molecular Structure and Bonding

Molecular Geometry and Electronic Structure

N-Sulfinylaniline adopts a nearly planar molecular geometry with the phenyl ring and S=N=O moiety lying in approximately the same plane. X-ray crystallographic analysis confirms a C–S=N=O dihedral angle of –1.60°, indicating effective conjugation between the aromatic system and the sulfinylimine group. The sulfur atom exhibits sp² hybridization with bond angles approximating 120° around the sulfur center. The S=N bond length measures 1.465 Å, while the S=O bond extends 1.445 Å, both values intermediate between single and double bonds due to significant π-delocalization across the N=S=O system.

The electronic structure features extensive delocalization with the highest occupied molecular orbital (HOMO) primarily localized on the nitrogen and sulfur atoms, while the lowest unoccupied molecular orbital (LUMO) demonstrates significant antibonding character between sulfur and oxygen. This electronic distribution results in a molecular dipole moment of approximately 4.2 D, oriented along the S=O bond vector. The compound exhibits two major resonance structures: one with formal positive charge on nitrogen and negative charge on oxygen (Ph-N⁺=S-O⁻), and another with charge separation between sulfur and nitrogen (Ph-N-S⁺=O⁻).

Chemical Bonding and Intermolecular Forces

The bonding in N-sulfinylaniline involves significant π-delocalization across the N=S=O functionality, with bond orders calculated as approximately 1.5 for both S-N and S-O bonds. The S-N bond energy is estimated at 280 kJ/mol, while the S=O bond demonstrates greater strength at 523 kJ/mol. Comparative analysis with related compounds shows bond lengths intermediate between sulfoxides (S=O ~1.47 Å) and sulfur diimides (S=N ~1.54 Å).

Intermolecular forces are dominated by dipole-dipole interactions due to the substantial molecular dipole moment. Van der Waals forces contribute significantly to the liquid-state properties, while hydrogen bonding is minimal due to the absence of conventional hydrogen bond donors. The compound's polarity results in moderate solubility in polar organic solvents such as dichloromethane and acetonitrile, but limited solubility in non-polar hydrocarbons and water.

Physical Properties

Phase Behavior and Thermodynamic Properties

N-Sulfinylaniline presents as a straw-colored to yellowish oil at room temperature with a characteristic pungent odor. The compound exhibits a density of 1.236 g/mL at 20 °C. The boiling point occurs at 88–95 °C under reduced pressure (17–20 mmHg), while decomposition precedes boiling at atmospheric pressure. The melting point is not well-defined due to the compound's tendency to supercool and decompose upon solidification.

Thermodynamic parameters include an estimated heat of vaporization of 45.2 kJ/mol and heat of formation of –125.4 kJ/mol. The specific heat capacity measures approximately 1.82 J/g·K in the liquid state. The refractive index is 1.612 at 20 °C and 589 nm wavelength. The compound demonstrates moderate thermal stability, decomposing above 150 °C through cleavage of the S-N bond and evolution of sulfur oxides.

Spectroscopic Characteristics

Infrared spectroscopy reveals characteristic vibrations at 1135 cm⁻¹ (S=O stretch), 1290 cm⁻¹ (S=N stretch), and 1495 cm⁻¹ (N-S stretch). The S=O stretching frequency is significantly reduced compared to typical sulfoxides (1050-1070 cm⁻¹) due to conjugation with the nitrogen atom. Proton NMR spectroscopy shows aromatic protons as a multiplet at δ 7.3-7.5 ppm and no aliphatic protons. Carbon-13 NMR displays signals at δ 121.5, 129.2, 130.1, and 135.4 ppm for the aromatic carbons, with the ipso carbon appearing at δ 150.2 ppm due to deshielding by the electron-withdrawing NSO group.

UV-Vis spectroscopy demonstrates absorption maxima at 255 nm (ε = 12,400 M⁻¹cm⁻¹) and 295 nm (ε = 8,200 M⁻¹cm⁻¹) corresponding to π→π* transitions of the aromatic system and n→π* transitions of the N=S=O chromophore. Mass spectrometry exhibits a molecular ion peak at m/z 139 with characteristic fragmentation patterns including loss of oxygen (m/z 123), loss of SO (m/z 93), and formation of the phenyl cation (m/z 77).

Chemical Properties and Reactivity

Reaction Mechanisms and Kinetics

N-Sulfinylaniline demonstrates significant reactivity as an electrophile due to the electron-deficient sulfur center. The compound functions as an excellent dienophile in Diels-Alder reactions, with second-order rate constants typically ranging from 10⁻³ to 10⁻¹ M⁻¹s⁻¹ depending on the diene partner. Cycloadditions proceed through concerted [4+2] mechanisms with activation energies of 60-80 kJ/mol. The compound also undergoes nucleophilic addition at sulfur, with thiols and amines adding across the S=N bond to form sulfenamides and other derivatives.

Hydrolysis occurs readily in aqueous media, with a half-life of approximately 45 minutes at pH 7 and 25 °C, yielding aniline and sulfurous acid. The hydrolysis mechanism involves nucleophilic attack of water at sulfur followed by cleavage of the S-N bond. Thermal decomposition follows first-order kinetics with an activation energy of 120 kJ/mol, producing sulfur dioxide and aniline as primary decomposition products.

Acid-Base and Redox Properties

N-Sulfinylaniline exhibits weak basic character with protonation occurring at the oxygen atom rather than nitrogen, yielding a stabilized sulfinylammonium species. The proton affinity of the oxygen atom is estimated at 825 kJ/mol. The compound demonstrates limited stability in acidic conditions, decomposing rapidly below pH 3. In basic media, hydrolysis accelerates significantly with a half-life of less than 5 minutes at pH 10.

Redox properties include reduction potentials of –0.85 V vs. SCE for one-electron reduction and –1.25 V for two-electron reduction. Oxidation occurs at +1.35 V vs. SCE, leading to formation of N-sulfonylaniline derivatives. The compound functions as a mild oxidizing agent toward thiols and other reducing agents, with standard reduction potential of the NSO/NSH₂ couple estimated at +0.45 V.

Synthesis and Preparation Methods

Laboratory Synthesis Routes

The primary laboratory synthesis involves reaction of aniline with thionyl chloride in a 3:1 molar ratio: 3 C₆H₅NH₂ + SOCl₂ → C₆H₅NSO + 2 [C₆H₅NH₃]Cl. The reaction proceeds through initial formation of N-thionylaniline intermediates followed by elimination of hydrogen chloride. Optimal conditions employ anhydrous diethyl ether or dichloromethane as solvent at 0–5 °C, with careful addition of thionyl chloride to control exothermicity. Typical yields range from 65–75% after purification by vacuum distillation.

Alternative synthetic routes include oxidation of N-sulfenylaniline derivatives with meta-chloroperbenzoic acid or hydrogen peroxide, and reaction of aniline with sulfur monoxide generated in situ. The latter method provides higher purity product but requires specialized apparatus for handling reactive gas mixtures. Purification typically involves fractional distillation under reduced pressure (17–20 mmHg) with collection of the fraction boiling at 88–95 °C.

Analytical Methods and Characterization

Identification and Quantification

Gas chromatography with flame ionization detection provides effective separation and quantification of N-sulfinylaniline, with retention indices of 1450–1500 on non-polar stationary phases. High-performance liquid chromatography employing C18 reverse-phase columns with UV detection at 255 nm offers alternative quantification methods with detection limits of 0.1 μg/mL. Thin-layer chromatography on silica gel with ethyl acetate/hexane (1:4) mobile phase yields Rf values of 0.45–0.50.

Purity Assessment and Quality Control

Purity assessment typically involves determination of aniline content by derivatization with fluorescamine or other amine-specific reagents, with commercial specifications requiring less than 0.5% free aniline. Common impurities include N,N'-thionydianiline, sulfonyl derivatives, and chlorinated byproducts. Quality control standards require minimum purity of 98% by GC analysis, with water content below 0.1% by Karl Fischer titration.

Applications and Uses

Industrial and Commercial Applications

N-Sulfinylaniline serves as a versatile intermediate in the production of sulfonamides, sulfinamides, and other organosulfur compounds with applications in agrochemicals and specialty chemicals. The compound functions as a dienophile in industrial Diels-Alder reactions for the synthesis of complex heterocyclic systems. Its ligand properties find application in coordination chemistry, particularly in the formation of transition metal complexes with catalytic activity.

Research Applications and Emerging Uses

In research settings, N-sulfinylaniline provides a convenient source of the NSO functional group for the synthesis of more complex sulfinylamine derivatives. Recent investigations explore its use as a building block for sulfur-nitrogen polymers and as a precursor to novel heterocyclic systems. Emerging applications include its use as a transfer agent for the NSO group in synthetic transformations and as a template for developing new ligands in coordination chemistry.

Historical Development and Discovery

The chemistry of sulfinylamines began developing in the early 20th century with the pioneering work of Lecher and colleagues on sulfur-nitrogen compounds. N-Sulfinylaniline was first characterized in detail during the 1950s as part of systematic investigations into the reactions of thionyl halides with amines. Structural elucidation advanced significantly with the application of infrared spectroscopy, which confirmed the N=S=O functionality. X-ray crystallographic studies in the 1970s provided definitive evidence for the planar structure and bond characteristics. The compound's dienophilic properties were extensively explored during the 1980s, leading to its established role in synthetic methodology.

Conclusion

N-Sulfinylaniline represents a structurally interesting and synthetically useful organosulfur compound with distinctive electronic properties derived from the conjugated N=S=O system. Its nearly planar geometry and significant dipole moment influence both physical properties and chemical reactivity. The compound serves as a valuable synthetic intermediate and dienophile with applications spanning organic synthesis, coordination chemistry, and materials science. Future research directions likely include expanded applications in asymmetric synthesis, development of novel NSO-containing materials, and exploration of its coordination chemistry with emerging catalytic systems.